Activity

Intermediate

Decays per second in becquerels.

Key relationA = λN

A = −dN/dt = λN in becquerels (1 Bq = 1 decay/s); the curie is 3.7×10¹⁰ Bq. Activity decays with the same half-life.

Worked example

1 g of radium: about 1 Ci.

Common mistake

Not equal to the number of nuclei.

In more detail

Activity is the rate of decay of a radioactive sample, A = λN, measured in becquerels. One becquerel is one decay per second.

Radiation safety regulations set limits on activity of sources.
Medical doses are specified in activity.
Health physicists measure the activity of a radioactive source in becquerels to judge its hazard.

What is the unit of activity?

The becquerel, and the older unit is the curie.

How does activity change over time?

It decreases exponentially, with the same half-life as the number of nuclei.

Is activity the same as dose?

No. Activity counts decays per second, while dose measures the energy absorbed by tissue, which is a different quantity.